Using Hot Air to Let off Steam

Forced-air heating systems could meet cold-weather demands.

According to Table 3.1 of "Recommended Concrete Temperatures" in ACI 306, "Cold Weather Concreting," concrete should be about 5º F warmer for sections with a minimum dimension of less than 12 inches than for sections with a minimum dimensions of 12-36 inches. Five degrees is a lot to make up on large pours. Some innovative producers are looking to forced-air aggregate heating.

Engineering data suggest that some fresh concrete ingredients have a greater influence on concrete temperatures than others. Heating the aggregates seems to be the producer's best choice next to heating the water.

Producers should try to raise the average maximum temperature of aggregate to about 60º F and sand to 105º F. Just as with mixing water, maximum temperature levels exist for aggregates as well. ACI 306 recommends that aggregate temperature not exceed 212º F at any one spot and that it does not exceed 150º F on average.

Heating aggregates is nothing new. Traditionally, producers have employed boiler steam for this purpose. But with steam comes several operating problems.

First, aggregate doesn't retain its elevated temperature for a long time. Steam mist tends to be very localized, creating pockets of aggregate with varying temperatures. And steam injection systems are high-maintenance items. Nozzles require almost daily monitoring, condensation water must be gathered before the material reaches the aggregate batcher, and the presence of water in steel bins tends to cause corrosion.

Unlike the use of steam, forced warm air allows producers to set the heating level to defrost and warm the aggregate just enough to eliminate ice and lumps. And on those days when higher temperatures are needed, the warm air raises the temperature of the entire bin to a higher level.

There are other benefits from using forced air. Aggregate temperature is more consistent. Because the system is warming the contents of the entire bin, the aggregates closest to the steam's discharge point won't have a disproportionately high temperature. In addition, because no water is introduced to the aggregate, it's drier, and dry material is easier to batch.

The drier aggregate also can help economize mix designs. By using a drier, warmed material, producers can better control water-cement ratio and set time with use of nonchloride accelerators.

The article also includes information that explains the effect of raising the temperature of various components on the concrete mix temperature.